Method of making a bi-directional read/program non-volatile floating gate memory cell

ABSTRACT

A bi-directional read/program non-volatile memory cell and array is capable of achieving high density. Each memory cell has two spaced floating gates for storage of charges thereon. The cell has spaced apart source/drain regions with a channel therebetween, with the channel having three portions. One of the floating gate is over a first portion; another floating gate is over a second portion, and a gate electrode controls the conduction of the channel in the third portion between the first and second portions. An independently controllable control gate is insulated from each of the source/drain regions, and is also capacitively coupled to the floating gate. The cell programs by hot channel electron injection, and erases by Fowler-Nordheim tunneling of electrons from the floating gate to the gate electrode. Bi-directional read permits the cell to be programmed to store bits, with one bit in each floating gate. The independently controllable control gates permit an array of such memory cells to operate in a NAND configuration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/409,407 filed on Apr. 7, 2003, the subject matter of whichis incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a bi-directional read/programnon-volatile memory cell, that uses a floating gate for storage ofcharges. More particularly, the present invention relates to suchnon-volatile memory cell that has independent controllable control gatesand is capable of storing a plurality of bits in a single cell and anarray of such cells, and a method of manufacturing.

BACKGROUND OF THE INVENTION

Uni-directional read/program non-volatile memory cells using floatinggate for storage are well known in the art. See for example, U.S. Pat.No. 5,029,130. Typically, each of these types of memory cells uses aconductive floating gate to store one bit, i.e. either the floating gatestores charges or it does not. The charges stored on a floating gatecontrol the conduction of charges in a channel of a transistor. In adesire to increase the storage capacity of such non-volatile memorycells, the floating gate of such memory cell is programmed to store somecharges, with the different amount of charges stored being determinativeof the different states of the cell, thereby causing a plurality of bitsto be stored in a single cell. The problem with programming a cell toone of a multilevel state and then reading such a state is that theamount of charge stored on the floating gate differentiating one statefrom another must be very carefully controlled.

Bi-directional read/program non-volatile memory cells capable of storinga plurality of bits in a single cell are also well known in the art.See, for example, U.S. Pat. No. 6,011,725. Typically, these types ofmemory cells use an insulating trapping material, such as siliconnitride, which is between two other insulation layers, such as silicondioxide, to trap charges. The charges are trapped near the source/drainalso to control the conduction of charges in a channel of a transistor.The cell is read in one direction to determine the state of chargestrapped near one of the source/drain regions, and is read in theopposite direction to determine the state of charges trapped near theother source/drain region. Hence, these cells are read and programmedbi-directionally. The problem with these types of cells is that toerase, holes or charges of the opposite conductivity must also be“programmed” or injected into the trapping material at precisely thesame location where the programming charges were initially trapped inorder to “neutralize” the programming charges. Since the programmingcharges and the erase charges are injected into a non-conductivetrapping material, the charges do not move as in a conductive material.Therefore, if there is any error in injecting the erase charges to thelocation of the programming charges, the erase charges will notneutralize the programming charges, and the cell will not be completelyerased. Moreover, to inject the erase charges, the cell must be erasedbi-directionally, thereby increasing the time required for erasure ofone cell.

Hence there is a need for a non-volatile memory cell and array thatovercomes these problems.

SUMMARY OF THE INVENTION

In the present invention, a non-volatile memory cell for the storage ofa plurality of bits comprises a substantially single crystallinesemiconductive material, such as single crystalline silicon, of a firstconductivity type. A first region of a second conductivity type,different from the first conductivity type is in the substrate. A secondregion of the second conductivity type is also in the substrate, spacedapart from the first region. A channel region, having a first portion, asecond portion and a third portion, connects the first and secondregions for the conduction of charges. A dielectric is on the channelregion. A first floating gate is on the dielectric, spaced apart fromthe first portion of the channel region. The first portion of thechannel region is adjacent to the first region. The first floating gateis for the storage of at least one of the plurality of bits. A secondfloating gate is on the dielectric, spaced apart from the second portionof the channel region. The second portion of the channel region isadjacent to the second region. The second floating gate is for thestorage of at least another of the plurality of bits. A gate electrodeis on the dielectric, spaced apart from the third portion of the channelregion. The third portion of the channel region is between the firstportion and the second portion. A first gate electrode is insulated fromthe first region and is also capacitively coupled to the first floatinggate. A second gate electrode is insulated from the second region and isalso capacitively coupled to the second floating gate.

The present invention also relates to an array of the foregoingdescribed non-volatile memory cells, and a method of making thenon-volatile memory cell and the array.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view of a semiconductor substrate used in the firststep of the method of present invention to form isolation regions.

FIG. 1B is a cross sectional view of the structure taken along the line1B—1B showing the initial processing steps of the present invention.

FIG. 1C is a top view of the structure showing the next step in theprocessing of the structure of FIG. 1B, in which isolation regions aredefined.

FIG. 1D is a cross sectional view of the structure in FIG. 1C takenalong the line 1D—1D showing the isolation trenches formed in thestructure.

FIG. 1E is a cross sectional view of the structure in FIG. 1D showingthe formation of isolation blocks of material in the isolation trenches.

FIG. 1F is a cross sectional view of the structure in FIG. 1E showingthe final structure of the isolation regions.

FIGS. 2A–2P are cross sectional views of the semiconductor structure inFIG. 1F taken along the line 2A—2A showing in sequence the steps in theprocessing of the semiconductor structure in the formation of anon-volatile memory array of floating gate memory cells of the presentinvention.

FIG. 3 is a schematic circuit diagram of the memory cell array of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The method of the present invention is illustrated in FIGS. 1A to 1F and2A to 2P, which show the processing steps in making the memory cellarray of the present invention. The method begins with a semiconductorsubstrate 10, which is preferably of P type and is well known in theart. The thickness of the layers described below will depend upon thedesign rules and the process technology generation. What is describedherein is for the 0.10 micron process. However, it will be understood bythose skilled in the art that the present invention is not limited toany specific process technology generation, nor to any specific value inany of the process parameters described hereinafter.

Isolation Region Formation

FIGS. 1A to 1F illustrate the Well known STI method of forming isolationregions on a substrate. Referring to FIG. 1A there is shown a top planview of a semiconductor substrate 10 (or a semiconductor well), which ispreferably of P type and is well known in the art. First and secondlayers of material 12 and 14 are formed (e.g. grown or deposited) on thesubstrate. For example, first layer 12 can be silicon dioxide(hereinafter “oxide”), which is formed on the substrate 10 by any wellknown technique such as oxidation or oxide deposition (e.g. chemicalvapor deposition or CVD) to a thickness of approximately 60–150angstroms. Second layer 14 can be silicon nitride (hereinafter“nitride”), which is formed over oxide layer 12 preferably by CVD to athickness of approximately 1000–2000 angstroms. FIG. 1B illustrates across-section of the resulting structure.

Once the first and second layers 12/14 have been formed, suitable photoresist material 16 is applied on the nitride layer 14 and a masking stepis performed to selectively remove the photo resist material fromcertain regions (stripes 18) that extend in the Y or column direction,as shown in FIG. 1C. Where the photo-resist material 16 is removed, theexposed nitride layer 14 and oxide layer 12 are etched away in stripes18 using standard etching techniques (i.e. anisotropic nitride and oxideetch processes) to form trenches 20 in the structure. The distance Wbetween adjacent stripes 18 can be as small as the smallest lithographicfeature of the process used. A silicon etch process is then used toextend trenches 20 down into the silicon substrate 10 to a depth ofapproximately 500–4000 angstroms, as shown in FIG. 1D. Where the photoresist 16 is not removed, the nitride layer 14 and oxide layer 12 aremaintained. The resulting structure illustrated in FIG. 1D now definesactive regions 22 interlaced with isolation regions 24.

The structure is further processed to remove the remaining photo resist16. Then, an isolation material such as silicon dioxide is formed intrenches 20 by depositing a thick oxide layer, followed by aChemical-Mechanical-Polishing or CMP etch (using nitride layer 14 as anetch stop) to remove the oxide layer except for oxide blocks 26 intrenches 20, as shown in FIG. 1E. The remaining nitride and oxide layers14/12 are then removed using nitride/oxide etch processes, leaving STIoxide blocks 26 extending along isolation regions 24, as shown in FIG.1F.

The STI isolation method described above is the preferred method offorming isolation regions 24. However, the well known LOCOS isolationmethod (e.g. recessed LOCOS, poly buffered LOCOS, etc.) couldalternately be used, where the trenches 20 may not extend into thesubstrate, and isolation material may be formed on the substrate surfacein stripe regions 18. FIGS. 1A to 1F illustrate the memory cell arrayregion of the substrate, in which columns of memory cells will be formedin the active regions 22 which are separated by the isolation regions24. It should be noted that the substrate 10 also includes at least oneperiphery region in which control circuitry is formed that will be usedto operate the memory cells formed in the memory cell array region.Preferably, isolation blocks 26 are also formed in the periphery regionduring the same STI or LOCOS process described above.

Memory Cell Formation

The structure shown in FIG. 1F is further processed as follows. FIGS. 2Ato 2Q show the cross sections of the structure in the active regions 22from a view orthogonal to that of FIG. 1F (along line 2A—2A as shown inFIGS. 1C and 1F).

An insulation layer 30 (preferably oxide) is first formed over thesubstrate 10, as shown in FIG. 2A. The active region 22 portion of thesubstrate 10 can be doped at this time for better independent control ofthe cell array portion of the memory device relative to the peripheryregion. Such doping is often referred to as a Vt implant or cell wellimplant, and is well known in the art. During this implant, theperiphery region is protected by a photo resist layer, which isdeposited over the entire structure and removed from just the memorycell array region of the substrate.

Next, a thick layer of hard mask material 32 such as nitride is formedover oxide layer 30 (e.g. ˜3500 Å thick). A plurality of parallel secondtrenches 34 are formed in the nitride layer 32 by applying a photoresist (masking) material on the nitride layer 32, and then performing amasking step to remove the photo resist material from selected parallelstripe regions. An anisotropic nitride etch is used to remove theexposed portions of nitride layer 32 in the stripe regions, leavingsecond trenches 34 that extend down to and expose oxide layer 30. Afterthe photo resist is removed, an anisotropic oxide etch is used to removethe exposed portions of oxide layer 30 and extend second trenches 34down to the substrate 10. A silicon anisotropic etch process is thenused to extend second trenches 34 down into the substrate 10 in each ofthe active regions 22 (for example, down to a depth of approximately onefeature size deep, e.g. about 0.15 um deep with 0.15 um technology).Alternately, the photo resist can be removed after trenches 34 areformed into the substrate 10. The resulting active region 22 is shown inFIG. 2B.

A layer of insulation material 36 is next formed (preferably using athermal oxidation process) along the exposed silicon in second trenches34 that forms the bottom and lower sidewalls of the second trenches 34(e.g. ˜70 Å to 120 Å thick). A thick layer of polysilicon 38(hereinafter “poly”) is then formed over the structure, which fillssecond trenches 34. Poly layer 38 can be doped (e.g. n+) by ion implant,or by an in-situ process. The resulting active region 22 is shown inFIG. 2C.

A poly etch process (e.g. a CMP process using nitride layer 32 as anetch stop) is used to remove poly layer 38 except for blocks 40 of thepolysilicon 38 left remaining in second trenches 34. A controlled polyetch is then used to lower the height of poly blocks 40, where the topsof poly blocks 40 are disposed above the surface of the substrate, butbelow the tops of STI blocks 26 in the isolation regions 24, as shown inFIG. 2D.

Another poly etch is then performed to create sloped portions 42 on thetops of poly blocks 40 (adjacent the second trench sidewalls). Nitridespacers 44 are then formed along the second trench sidewalls and overthe sloped portions 42 of poly blocks 40. Formation of spacers is wellknown in the art, and involves the deposition of a material over thecontour of a structure, followed by an anisotropic etch process, wherebythe material is removed from horizontal surfaces of the structure, whilethe material remains largely intact on vertically oriented surfaces ofthe structure. Spacers 44 can be formed of any dielectric material, suchas oxide, nitride, etc. In the present embodiment, insulating spacers 44are formed by depositing a layer of nitride over the entire structure,followed by an anisotropic nitride etch process, such as the well knownReactive Ion Etch (RIE), to remove the deposited nitride layer exceptfor spacers 44. The resulting active region 22 is shown in FIG. 2E. Itshould be noted that the formation of nitride spacers 44 is optional, asthe spacers 44 are used to enhance the sharpness of the tips formed bythe sloped portions 42 of poly blocks 40. Thus, FIGS. 2F–2Q show theremaining processing steps without the optional nitride spacers 44.

A thermal oxidation process is then performed, which oxidizes theexposed top surfaces of the poly blocks 40 (forming oxide layer 46thereon), as shown in FIG. 2F. Oxide spacers 48 (shown in FIG. 2G) arethen formed along the sidewalls of the second trenches 34 by depositingoxide over the structure (e.g. approximately 350 Å thickness) followedby an anisotropic oxide etch. The oxide etch also removes the centerportion of oxide layer 46 in each of the second trenches 34. Theresulting active region 22 is shown in FIG. 2G.

An anisotropic poly etch is next performed, which removes the centerportions of the poly blocks 40 that are not protected by oxide spacers48, leaving a pair of opposing poly blocks 40 a in each of the secondtrenches 34, as shown in FIG. 2H. An insulation deposition andanisotropic etch-back process is then used to form an insulation layer50 along the exposed sides of poly blocks 40 a inside second trenches 34(shown in FIG. 21). The insulation material could be any insulationmaterial (e.g. ONO—oxide/nitride/oxide, or other high dielectricmaterials). Preferably, the insulation material is oxide, so that theoxide deposition/etch process also thickens the oxide spacers 48 andresults in the removal of the exposed portions of oxide layer 36 at thebottom of each second trench 34 to expose the substrate 10, as shown inFIG. 2J. In addition, when the oxide layer 36 at the bottom of eachtrench 34 is removed, the process also removes the oxide in the STIbetween adjacent columns of active regions 22 in the trench 34.

Suitable ion implantation (and possible anneal) is then made across thesurface of the structure to form first (source) regions 52 in theexposed substrate portions at the bottom of second trenches 34. Thesource regions 52 are self-aligned to the second trenches 34 and form acontinuous row that is substantially perpendicular to the column of theactive regions 22, and have a second conductivity type (e.g. N type)that is different from a first conductivity type of the substrate (e.g.P type). The ions have no significant effect on the nitride layer 32.The resulting active region 22 is shown in FIG. 2K.

An oxidation deposition step follows and fills the bottom of each trench34 with a layer of oxide 35 of approximately at least 100 angstroms, andno thicker than the height of the to-be-formed-floating gate-poly block40 so that capacitive coupling between the to-be-deposited-and formedcontrol gate 54 and the poly block 40 can occur. This is then followedby a poly deposition step, followed by a poly CMP etch (using thenitride layer 32 as an etch stop) are used to fill second trenches 34with poly blocks 54, as shown in FIG. 2L. Thus, the poly 54 fills eachtrench 34 in a continuous row. A nitride etch follows, which removesnitride layer 32, and exposes upper edges of the poly blocks 40 a. Atunnel oxide layer 56 is next formed on the exposed upper edges of polyblocks 40 a, either by thermal oxidation, oxide deposition, or both.This oxide formation step also forms an oxide layer 58 on the exposedtop surfaces of poly blocks 54, as well as possibly thickening oxidelayer 30 over substrate 10. Optional Vt implantation in the peripheryregion can be performed at this time by masking off the active regions22. The resulting active region 22 is shown in FIGS. 2M and 2N.

The oxide layer 30 serves as the gate oxide for both the memory cells inthe active regions, and the control circuitry in the periphery region.For each device, the thickness of the gate oxide dictate's its maximumoperating voltage. Thus, if it is desired that some of the controlcircuitry operate at a different voltage than the memory cells or otherdevices of the control circuitry, then the thickness of the gate oxide32 can be modified at this point in the process. In way of example butnot limitation, photo resist 60 is formed over the structure, followedby a masking step for selectively removing portions of the photo resistin the periphery region to expose portions of oxide layer 30. Theexposed portions of oxide layer 30 can be thinned (e.g. by using acontrolled etch) or replaced (e.g. by an oxide etch and oxidedeposition) with oxide layer 30 a having the desired thickness, asillustrated in FIG. 2O.

After removal of photo resist 60, a poly deposition step is used to forma poly layer 62 over the structure (e.g. approximately 500 Å thick).Photo resist deposition and masking steps follow to form strips of polylayer 62 that are spaced apart from one another each over an activeregion 22. The resulting active region 22 is shown in FIG. 2P. Each polylayer 62 functions as a word line for the memory array.

As shown in FIG. 2P, the process of the present invention forms an arrayof memory cells, with each memory cell 15 being between a pair of spacedapart source/drain regions 52(a,b) (those skilled in the art wouldappreciated that the term source and drain may be interchanged duringoperation.) A non-planar channel region connects the two source regions52(a,b), with the channel region having three portions: a first portion,a second portion and a third portion. The first portion of the channelregion is along one of the sidewall of one of the trenches 34, and isadjacent to the first source region 52 a. The second portion of thechannel region is along one of the sidewall of the other trench 34, andis adjacent to the second source region 52 b. A third portion of thechannel region is between the first portion and the second portion andis substantially along the top surface of the substrate 10. A dielectriclayer is over the channel region. Over the first portion of the channelregion, the dielectric is the layer 36 a. Over the second portion of thechannel, the dielectric is the layer 36 b. Over the third portion of thechannel region, the dielectric is the layer 30. A first floating gate 40a is on the layer 36 a, and is over the first portion of the channelregion, which is adjacent to the first source region 52 a. A secondfloating gate 40 b is on the layer 36 b, and is over the second portionof the channel region, which is adjacent to the second source region 52b. A gate electrode 62, formed by the poly layer 62, is over thedielectric layer 30 and is over the third portion of the channel region.A first control gate 54 a is insulated from the first source region 52a, and is capacitively coupled to the first floating gate 40 a. A secondcontrol gate 54 b is insulated from the second source region 52 b, andis capacitively coupled to the second floating gate 40 b. Further, eachof the floating gates 40 a and 40 b is substantially perpendicular tothe gate electrode 62 and to the surface of the substrate 10. Finally,each source region, e.g. first source region 52 a, and its associatedcontrol gate, e.g. first control gate 54 a is shared with an adjacentmemory cell 15 in the same active region 22.

The floating gates 40(a,b) are disposed in trenches 34, with eachfloating gate facing and insulated from a portion of the channel region.Further, each floating gate 40(a,b) includes an upper portion thatextends above the substrate surface and terminates in an edge that facesand is insulated from one of the gate electrodes 62, thus providing apath for Fowler-Nordheim tunneling through oxide layer 56. Each controlgate 54 extends along and are insulated (by oxide layer 50) fromfloating gates 44, for enhanced voltage coupling therebetween.

With respect to the plurality of memory cells 15 that form an array, theinterconnection is as follows. For memory cells 15 that are in the samecolumn, i.e. in the same active region 22, the word line 62 that formsthe gate electrode for each memory cell 15 is extended in the Ydirection to each of the memory cells 15. For memory cells 15 that arein the same row, i.e. across the active regions 22 and the STI 26, thesource lines 52(a,b) and the associated control gates 54(a,b) extendcontinuously in the X direction to each of those memory cells 15.Finally, as can be seen from the foregoing, memory cells 15 in adjacentrows, share the same source region 52 and the same associated controlgate 54. Each of the memory cells 15 has five independently controllableterminals: word line 62, control gates 54(a,b) and source regions52(a,b).

As will be appreciated by those skilled in the art, lines 52 a, 52 b, 52c etc, are buried diffusion lines, and contacts must be made to thoselines outside of the array of memory cells. One approach is to use apoly block 54, similar to the control gate 54, however, with the polyblock 54 electrically contacting the buried diffusion lines 52 a, 52 b,52 c etc. outside of the array. Further, the poly block 54 whichcontacts the buried diffusion lines 52 a, 52 b, 52 c, etc. outside ofthe array, must not be in electrical contact with the independentcontrol gate 54 that is in the array.

Memory Cell Operation

The operation of the memory cell 15 shown in FIG. 2P will now bedescribed.

Erase

The memory cell 15 is erased by applying 0 volts to the control gates54(a,b), and 0 volts to the source regions 52(a,b). Since the samevoltage is applied to both source regions 52(a,b), no charges willconduct in the channel region. Furthermore, because the control gates54(a,b) are highly capacitively coupled to the floating gates 40(a,b),the floating gates 40(a,b) will experience a low voltage. A voltage ofbetween 8 to 12 volts is applied to the word line 62. This causes alarge voltage differential between the floating gates 40(a,b) and theword line 62. Any electrons stored on the floating gates 40(a,b) arepulled by the positive voltage applied to the word line 62, and throughthe mechanism of Fowler-Nordheim tunneling, the electrons are removedfrom the floating gates 40(a,b), and tunnel through the tunneling oxide56 onto the word line 62. This mechanism of poly-to-poly tunneling forerase is set forth in U.S. Pat. No. 5,029,130, whose disclosure isincorporated herein in its entirety by reference.

Programming

Programming of the memory cell 15 can occur in one of two mechanisms:either the first floating gate 40 a is programmed or the second floatinggate 40 b is programmed. Let us first discuss the action of programmingthe first floating gate 40 a, i.e. storage of electrons on the firstfloating gate 40 a. The first source region 52 a is held at a positivevoltage of between 7 to 12 volts. The first control gate 54 a is held ata positive voltage of between 2 to 5 volts. The word line 62 is held ata positive voltage of 1–3 volts. The second control gate 54 b is held ata positive voltage of between 1–2.5 volts. The second source region 52 bis held at 0 volts. Because the second control gate 54 b is stronglycapacitively coupled to the second floating gate 40 b, the positivevoltage of 1–2.5 volts on the second control gate 54 b is sufficient toturn on the second portion of the channel region, even if the secondfloating gate 40 b is programmed, i.e. has electrons stored thereon. Thepositive voltage of 1–2 volts on the word line 62 is sufficient to turnon the third portion of the channel region. The positive voltage of10–15 volts on the first source region 52 a is sufficient to attract theelectrons in the channel. The positive voltage of 2 to 3 volts on thefirst control gate 54 a is sufficient to turn on the first portion ofthe channel region (because the first floating gate 40 a is erased).Thus, electrons will traverse in the channel region from the secondsource region 54 b to the first source region 54 a. However, at thejunction in the channel region where the channel region takessubstantially a 90 degree turn in the direction from the planar surfaceto the first trench 34 a, the electrons will experience a suddenincrease in voltage, caused by the positive high voltage of the firstsource region 54 a. This causes the electrons to be hot channel injectedonto the first floating gate 40 a. This mechanism of hot channelelectron injection for programming is set forth in U.S. Pat. No.5,029,130, whose disclosure is incorporated herein in its entirety byreference.

To program the second floating gate 40 b, the voltages applied to thefirst control gate 54 a, first source region 52 a are reversed fromthose applied to the second control gate 54 b, and second source region52 b.

Read

Reading of the memory cell 15 can occur in one of two mechanisms: eitherthe state of the first floating gate 40 a is read, or the state of thesecond floating gate 40 b is read. Let us first discuss the action ofreading the state of the second floating gate 40 b, whether electronsare stored on the second floating gate 40 b. The first source region 52a is held at a positive voltage of between 2 to 3.5 volts. The firstcontrol gate 54 a is held at a positive voltage of between 2 to 3 volts.The word line 62 is held at a positive voltage of 2–3.5 volts. Thesecond source region 52 b is held at 0 volts. The second control gate 54b is held at a positive voltage of between 1–2.5 volts. The positivevoltage of 2–3 volts on the first control gate 54 a, and the positivevoltage of 2–3.5 volts on the first source region 52 a are sufficient toturn on the first portion of the channel region, even if the firstfloating gate 40 a is programmed, i.e. has electrons stored thereon. Thepositive voltage of 1.5–2.5 volts on the word line 62 is sufficient toturn on the third portion of the channel region. The positive voltage ofbetween 1 to 2.5 volt on the second control gate 54 b is sufficient toturn on the second portion of the channel region only if the secondfloating gate 40 b is not programmed. In that event, electrons willtraverse in the channel region from the second source region 54 b to thefirst source region 54 a. However, if the second floating gate 40 b isprogrammed, then the positive voltage of between 1 to 2.5 volt is notsufficient to turn on the second portion of the channel region. In thatevent, the channel remains non-conductive. Thus, the amount of currentor the presence/absence of current sensed at the first source region 52a determines the state of programming of the second floating gate 40 b.

To read the first floating gate 40 a, the voltages applied to the firstcontrol gate 54 a and first source region 52 a are reversed from thoseapplied to the second control gate 54 b and second source region 52 b.

Memory Cell Array Operation

The operation of an array of memory cells 15 will now be described.Schematically, an array of memory cells is shown in FIG. 3. As shown inFIG. 3, an array of memory cells 15 comprises a plurality of memorycells arranged in a plurality of columns: 15 a(1–k), 15 b(1–k), and 15c(1–k) and in rows: 15(a–n)1, 15(a–n)2 and 15(a–n)3. The word line 62connected to a memory cell 15 is also connected to other memory cells 15in the same column. The first and second source regions 52 and the firstand second control gates 54 connected to a memory cell 15 are alsoconnected to other memory cells in the same row.

Erase

In the erase operation, memory cells 15 in the same column connected bythe common word line 62 are erased simultaneously. Thus, for example, ifit is desired to erase memory cells 15 in the column 15 b(1–n), the wordline 2 is held at between 8 to 12 volts. The unselected word lines 1 and3 are held at 0 volts. All the source region lines 52 and control gatelines 54 are held at 0 volts. In this manner all of the memory cells 15b(1–n) are erased simultaneously, while no erase disturbance occurs withrespect to the memory cells 15 in the other columns because all fiveterminals to the memory cells 15 in all the other columns are at groundvoltage.

Program

Let us assume that the first floating gate 40 a of the memory cell 15 b1 is to be programmed. Then based upon the foregoing discussion, thevoltages applied to the various lines are as follows: line 52 a is at apositive voltage of between 7 to 12 volts. Line 54 a is at a positivevoltage of between 2 to 5 volts. Line 2 is at a positive voltage ofbetween 1–3 volts. Line 54 b is held at a positive voltage of 1–2.5volts. Line 52 b is held at 0 volts. All the other unselected columnlines, i.e. lines 1 and 3 are at 0 volts. Similarly, all the other rowlines, such as 54 c, 54 d, and 52 c and 52 d are at 0 volts. The“disturbance” on the unselected memory cells 15 are as follows:

For the memory cells 15 in the unselected column, the application of 0volts to lines 1 and 3 means that none of the channel regions for thosememory cells 15 c(1–n) and 15 a(1–n) are turned on, because the thirdportion of the channel region (the portion to which the word line 1 and3 control) are not turned on. Thus, there is no disturbance. For thememory cell 15 b 2 which is in the same selected column, but in anunselected row, the application of 0 volts to line 54 c means that theportion of the channel region of the memory cell 15 b 2 which isadjacent to the source region 52 c will not be turned on. In that eventthe channel between the source region 52 c and the source region 52 bwill be turned off. Thus, little or no disturbance to memory cell 15 b 2would occur. Similarly for all other memory cells 15 in the selectedcolumn but unselected row, a portion of the channel region of thosememory cells will not be turned on due to the 0 volts being applied tothe unselected control gates 54.

To program the second floating gate 40 b, the voltages applied to thefirst control gate line 54 a, first source region line 52 a are reversedfrom those applied to the second control gate line 54 b, and secondsource region line 52 b. All the other lines will have the same voltagesas discussed for the programming of the first floating gate 40 a.

Read

Let us assume that the second floating gate 40 b of the memory cell 15 b1 is to be read. Then based upon the foregoing discussion, the voltagesapplied to the various lines are as follows: The source region line 52 ais held at a positive voltage of between 2 to 3.5 volts. The firstcontrol gate line 54 a is held at a positive voltage between 2 to 3volts. The word line 62 or line 2 is held at a positive voltage of 2–3.5volts. The second source region line 52 b is held at 0 volts. The secondcontrol gate line 54 b is held at a positive voltage of between 1–2.5volts.

The voltages applied to the unselected word lines 62 (lines 1 and 3) andthe unselected source regions lines 52 c and 52 d, and the unselectedcontrol gate lines 54 c and 54 d are all held at ground or 0 volts. The“disturbance” on the unselected memory cells 15 is as follows:

For the memory cells 15 in the unselected columns, the application of 0volts to lines 1 and 3 means that none of the channel regions for thosememory cells 15 c(1–k) and 15 a(1–k) is turned on. Thus, there is nodisturbance. For the memory cell 15 b 2 which is in the same selectedcolumn, but in an unselected row, the application of 0 volts to line 54c means that the portion of the channel region of the memory cell 15 b 2which is adjacent to the source region 52 c will not be turned on. Inthat event the channel region will be turned off. Thus, little or nodisturbance to memory cell 15 b 2 would occur. Similarly, for all theother memory cells in the same selected column but unselected rows,there will not be any disturbance.

To read the first floating gate 40 a, the voltages applied to the firstcontrol gate line 54 a, first source region line 52 a are reversed fromthose applied to the second control gate line 54 b, and second sourceregion line 52 b. All the other lines will have the same voltages asdiscussed for the reading of the second floating gate 40 b.

NAND Operation

One unique feature of an array of memory cells 15 of the presentinvention is the ability of the array to operate as a NAND device. ANAND device has a string of NVM connected in a serial fashion to asource of programming/read voltage. Let us assume that one string of NVMcells comprises: 15 b 1, 15 b 2, and 15 b 3 all in the same columnconnected by the same word line 62.

Erase

The erase operation for the string of NVM cells in the same string isthe same as that described previously for memory cells being erased inan array. Cells in the same column connected by the common word line 62are erased simultaneously. Thus, cells in the same NAND string areerased simultaneously.

Program

To program a particular cell, in a string of NVM cells, e.g. floatinggate 40 e of cell 15 b 3 in a string of NVM cells comprising cells 15b(1–3), the various voltages applied are as follows: A programmingvoltage, such as 7–12 volts, is first applied at buried diffusion line52 a. A “high” voltage is applied to the control gate 54 a, sufficientto “turn on” the channel adjacent the floating gate 40 a. A “high”voltage (1–3 volts) is applied to the word line 62 to “turn on” thechannel between the floating gate 40 a and floating gate 40 b. A “high”voltage (2–5 volts) is applied to the control gate 54 b to turn on thechannel adjacent to the floating gate 40 b. This causes the entirechannel region between the buried diffusion line 52 a and 52 b to beconducting. Buried diffusion line 52 b is held floating. This causes theprogramming voltage from diffusion line 52 a to be present at diffusionline 52 b. The “turning on” of the channel region for other cellscontinues until the programming voltage is at the buried diffusion line52 c. A ground voltage is applied to buried diffusion line 52 d, whichis at the other end of the chain of a string of NVM cells. A voltage of1–2.5 volts is applied to the control gate 54 d, which turns on thechannel adjacent to the floating gate 40 f. Since the word line 62 is ata high voltage to turn on the channel region between the floating gate40 f and floating gate 40 e, electrons traverse the channel region andare injected by hot channel electron injection onto the floating gate 40e.

To program the floating gate 40 f of memory cell 15 b 3, the programmingvoltage is first applied to the other end of the string of NVM cells,i.e. to diffusion line 52 d. Ground voltage is applied to diffusion line52 a, and through the mechanism previously discussed, the ground voltageis transferred to diffusion line 52 c, which then causes hot channelelectrons to program the floating gate 40 f.

Read

To read a particular cell, in a string of NVM cells, e.g. floating gate40 e of cell 15 b 3 in a string of NVM cells comprising cells 15 b(1–3),the various voltages applied are as follows: A read voltage of 2 to 3.5volts is applied to the diffusion line 52 d. Ground voltage is appliedto diffusion line 52 a. A positive voltage, such as 1.5–3.5 volts isapplied to the word line 62. A positive voltage such as 1 to 2.5 voltsis applied to each of the control gate 54 a, 54 b, and 54 c. Thediffusion lines 52 b and 52 c would receive the ground voltage fromdiffusion line 52 a. Electrons traversing from diffusion line 52 c to 52d would be read and would be determinative of the state of the floatinggate 40 e.

From the foregoing it can be seen that a novel, high densitynon-volatile memory cell, array and method of manufacturing isdisclosed. It should be appreciated that although the preferredembodiment has been described in which a single bit is stored in each ofthe two floating gates in a memory cell, it is also within the spirit ofthe present invention to store multi-bits on each one of the floatinggates in a single memory cell, thereby increasing further the density ofstorage.

1. A method of manufacturing an array of non-volatile memory cells in asubstantially single crystalline semiconductive substrate material of afirst conductivity type, wherein said array of non-volatile memory cellshas a plurality of non-volatile memory cells arranged in a plurality ofrows and columns in said semiconductive substrate material with eachcell for storing a plurality of bits, said method comprising: formingspaced apart isolation regions on said semiconductive substrate that aresubstantially parallel to one another and extend in said columndirection, with an active region between each pair of adjacent isolationregions, wherein said semiconductive substrate has a surface; forming aplurality of memory cells in each of the active regions, wherein eachmemory cell for the storage of a plurality of bits, wherein theformation of each of the memory cells includes: forming a first and asecond spaced apart trenches into the surface of the substrate, each ofsaid first and second trenches having a sidewall and a bottom wall;forming a first region and a second region in said bottom wall of saidfirst and second trenches, respectively, with said first region and saidsecond region being of a second conductivity type, different from saidfirst conductivity type, with a channel region for the conduction ofcharges connecting said first region and said second region; saidchannel region, having a first portion, a second portion and a thirdportion; forming a dielectric on said channel region; forming a firstfloating gate on said dielectric, in said first trench spaced apart fromsaid sidewall of said first trench and from said first portion of saidchannel region; said first portion of said channel region adjacent tosaid first region, said first floating gate for the storage of at leastone of said plurality of bits; forming a second floating gate on saiddielectric, in said second trench spaced apart from said sidewall ofsaid second trench and from said second portion of said channel region;said second portion of said channel region adjacent to said secondregion, said second floating gate for the storage of at least another ofsaid plurality of bits; forming a gate electrode on said dielectric,spaced apart from said third portion of said channel region, said thirdportion of said channel region between said first portion and saidsecond portion; forming a first gate electrode in said first trench,said first gate electrode insulated from said first region andcapacitively coupled to said first floating gate; and forming a secondgate electrode in said second trench, said second gate electrodeinsulated from said second region and capacitively coupled to saidsecond floating gate.
 2. The method of claim 1 wherein said step offorming said first gate electrode includes forming said first gateelectrode continuously in said row direction across a plurality ofcolumns.
 3. The method of claim 2 wherein said step of forming saidsecond gate electrode includes forming said second gate electrodecontinuously in said row direction across a plurality of columns.
 4. Themethod of claim 3 wherein said step of forming said first region andsaid second region includes forming said first region and said secondregion continuously in said row direction across a plurality of columns.5. The method of claim 3 wherein said step of forming said first regionand said second region includes forming said first region and saidsecond region in only one column of active region.
 6. The method ofclaim 3 wherein said step of forming said gate electrode includesforming said gate electrode continuously in said column direction acrossa plurality of memory cells.
 7. A method of manufacturing a non-volatilememory cell in a substantially single crystalline semiconductivesubstrate of a first conductivity type, wherein said non-volatile memorycell for storing a plurality of bits, said method comprising: forming afirst and a second spaced apart trenches into the surface of thesubstrate, each of said first and second trenches having a sidewall anda bottom wall; forming a first region and a second region in said bottomwall of said first and second trenches, respectively, with said firstregion and said second region being of a second conductivity type,different from said first conductivity type, with a channel region forthe conduction of charges connecting said first region and said secondregion; said channel region, having a first portion, a second portionand a third portion; forming a dielectric on said channel region;forming a first floating gate on said dielectric, in said first trenchspaced apart from said sidewall of said first trench and from said firstportion of said channel region; said first portion of said channelregion adjacent to said first region, said first floating gate for thestorage of at least one of said plurality of bits; forming a secondfloating gate on said dielectric, in said second trench spaced apartfrom said sidewall of said second trench and from said second portion ofsaid channel region; said second portion of said channel region adjacentto said second region, said second floating gate for the storage of atleast another of said plurality of bits; forming a gate electrode onsaid dielectric, spaced apart from said third portion of said channelregion, said third portion of said channel region between said firstportion and said second portion; forming a first gate electrode in saidfirst trench, said first gate electrode insulated from said first regionand capacitively coupled to said first floating gate; and forming asecond gate electrode in said second trench, said second gate electrodeinsulated from said second region and capacitively coupled to saidsecond floating gate.
 8. The method of claim 7 wherein said step offorming a gate electrode on said dielectric further includes forming aninsulation material between the gate electrode and said first and secondfloating gates with a thickness that permits Fowler-Nordheim tunnelingof electrons from said first and second floating gates to said gateelectrode.
 9. The method of claim 8 wherein said step of forming saidfirst and second floating gates includes forming said first and secondfloating gates above the substrate surface.